1. The Field of the Invention
The present invention generally relates to electron emitting devices. More particularly, the present invention relates to a cathode assembly that includes features directed to facilitating modifications to the density of the electron stream emitted by the cathode assembly.
2. The Relevant Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode generally comprises a metallic cathode head and a cathode cup disposed thereon. A rectangular slot formed in the cathode cup typically houses a filament that, when heated via an electrical current, emits a stream of electrons. The cathode is disposed within the vacuum enclosure, as is the anode, which is oriented to receive the electrons emitted by the cathode. The anode, which typically comprises a graphite substrate upon which is disposed a heavy metallic target surface, can be stationary within the vacuum enclosure, or can be rotatably supported by a rotor shaft and a rotor assembly. The rotary anode is typically spun using a stator that is circumferentially disposed about the rotor assembly, and is disposed outside of the vacuum enclosure. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic material, or a combination thereof, and is typically disposed within an outer housing.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. A high electric potential placed between the cathode (negative) and anode (positive) causes the electrons in the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. The point at which the electrons strike the target surface is referred to as the focal spot. Upon striking the focal spot, many of the electrons lose their kinetic energy, which causes the electrons or the target surface material to emit electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The target surface of the anode is angled with respect to the stream of electrons to minimize the size of the resultant x-ray beam, while maintaining a sufficiently sized focal spot. The x-ray beam produced by the target surface then passes through windows that are defined in the vacuum enclosure and outer housing. Finally, the x-ray beam is directed to the x-ray subject to be analyzed, such as a medical patient or a material sample.
As mentioned above, a typical cathode includes a cathode cup attached to a cathode head. A filament is disposed within a rectangular slot defined by the cathode cup. The filament typically comprises a wire made from tungsten or similar material that is uniformly wound about a mandrel to form a helix. The ends of the filament wire are electrically connected to leads disposed in the bottom of the cathode cup slot. In addition to housing the filament, the cathode cup also shapes the electrical field near the filament that is created by the high electric potential that exists between the cathode and the anode during tube operation. By shaping the electrical field, and thus affecting the strength of the electrical field between the cathode and anode, the cathode cup helps deflect electrons toward the focal spot on the anode target surface.
A recurrent challenge encountered in the operation of x-ray tubes concerns the uniformity of the electron stream emitted by the cathode, and the resultant uniformity of electron impacts upon the focal spot of the anode target surface. As mentioned earlier, electrons are produced during tube operation when a current is passed through the cathode filament, causing it to become heated. When the filament reaches a certain temperature, it begins to emit electrons by a process known as thermionic emission. During the thermionic emission process, however, a temperature gradient is established in the filament, wherein relatively higher temperatures are present in the middle region of the filament and relatively lower temperatures are present in the end regions of the filament. Because the rate at which electrons are produced by an electron-emitting medium is closely related to the temperature of the medium, the temperature gradient of the filament causes relatively more electrons to be produced by the middle region of the filament than by the end regions, thus creating an unevenly distributed cloud of electrons directly above the cathode.
The cloud of electrons described above generally resembles the shape of the filament. When considered from a viewpoint opposite the filament, the electron cloud appears relatively more populated with electrons near its middle region than near the ends of the cloud. The high electric potential present between the cathode and the anode causes the electrons in the electron cloud emitted by the cathode to accelerate toward the anode focal spot. During such acceleration, the electrons in the electron stream retain the uneven distribution described above. When the electron stream impacts the anode target surface, relatively more electron impacts occur on the area of anode focal spot corresponding to the middle region of the impacting electron stream than on the focal spot area corresponding to the ends of the stream. Undesirably, the uneven distribution of the impacting electrons results in an x-ray beam emitted by the x-ray tube having a similarly uneven distribution of x-rays across the beam when the electron beam is viewed in cross-section.
Unfortunately, such an x-ray beam produces images of relatively poor quality and detail. The performance of the x-ray tube is thus compromised, thereby necessitating the generation of additional x-ray images to compensate for the low quality images. The result is additional operating cost, waste of resources, and possible added risk to the human subject or operator of the x-ray generating device.
Some control over electron beam density may be achieved by way of an electron shield defining an aperture placed in the path of the uneven electron stream so as to selectively restrict the travel of portions of the unevenly distributed electron cloud. Such an approach is problematic for a variety of reasons however. First, the shield allows only a portion of the total number of electrons created by the filament to proceed to the focal spot, thus resulting in an inefficient use of x-ray tube power. Second, the surface of the shield alters the shaping of the electrical field near the cathode, which may undesirably affect electron acceleration toward the focal spot. Third, in order to stop the undesired electrons, the shield must dissipate their kinetic energy, which causes undesirable heating within the x-ray tube. Thus, additional heat removing structures or systems must be employed to compensate for the additional heating caused by the shield, which undesirably add to the cost and complexity of the tube.
A need therefore exists for a cathode assembly that includes features which permit adjustments to the density of the emitted electron beam. When disposed in an x-ray tube, the cathode should enable, among other things, production of x-ray beams having a substantially uniform cross-sectional density, thus permitting generation of higher quality images. Desirably, this need would be met without creating undesirable side effects, such as excessive tube heating.